1. Field of the Invention
The present invention is directed to voltage regulation in multivoltage supply applications, and in particular, for peripheral devices such as memory systems.
2. Description of the Related Art
Advances in integrated circuit (IC) technology often relate to decreasing the size and operating voltages of devices used to fabricate circuitry. Lower power devices generally translate into lower costs due to decreases in circuit size and power consumption. Currently, low-voltage integrated circuits having transistors operating in the three-volt range and lower are highly desirable. The three-volt ICs are gradually replacing the standard five-volt ICs due to their higher speed and higher integration densities. Moreover, the three-volt ICs consume less power than the traditional five-volt ICs. In battery operated devices, such as portable telephones and lap-top computers, low-voltage integrated circuits allows the devices to operate proportionally longer than devices requiring higher voltage for operation.
Improvements in processing technology have led to smaller transistor device sizes. This in turn has led to lower power supply requirements in each successive generation of transistor technology. For example, a 0.25 micron process device operates on a voltage of approximately 2.5 volts, a 0.18 micron process uses a 1.8 v (+/−10%) supply, a 0.15 micron process will use a 1.5 (+/−10%) volt supply, 0.13 micron technology a 1.2 v (+/−10%) supply, and so on.
The lower power requirements of newer semiconductor processes are not always compatible with legacy host devices. For example, older hosts may supply 5 or 3 volts to a memory system requiring only 3 or 1.8 volts, respectively. Computer peripheral devices must therefore be designed to accommodate connections from both 3.3 volt host supplies as well as older, higher voltage supplies. Devices constructed with newer technologies therefore allow for the possibility that the host device in which they are used will supply a higher voltage used with legacy products. Hence power level detection and voltage regulation techniques are used to provide the correct voltage to the memory device.
One such peripheral device which must make such adjustments is non-volatile memory which might be used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices, audio and video players and other appliances. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memory types.
Typical applications include the provision of one more arrays of storage elements in a memory card system, such as a memory card, which is then inserted in a host device. Both EEPROM and flash memory utilize arrays of storage elements to store one or more bits of data per element. Each element generally includes a floating gate that is positioned above and insulated from a channel region and a semiconductor substrate. The floating gate is positioned between source and drain regions. A control gate may be provided over and insulated from a floating gate. The threshold voltage of each memory transistor is controlled by the amount of charge that has retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before a transistor is turned on to permit conduction between its source entering is controlled by the level of charge on the floating gate. Many EEPROMs and flash memories have a floating gate that is used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two states. Such memory cells store one byte of data. Other EEPROMs and flash memory cells store multiple ranges of charge and, therefore, such memory cells can be programmed to store multiple states. Such memory cells store multiple bytes of data.
Non-volatile memory may be coupled to a host device in a number of ways. Peripheral adapters include integrated adapters coupled directly to a computers system bus and power supply, and peripherals adapted to connect to any of a number of industry standard external system connections, such as Universal Serial Bus or the IEEE 1394 Standard. These standard connections generally include a power and ground connection for the peripheral. Again, older connections provide a 5 v supply, not generally suited for newer peripherals with lower voltage transistors.
Generally, in order to couple peripherals manufactured with newer process technologies to legacy or hosts providing higher-supply voltages, step-down voltage regulators are used. Where the regulator is integrated with the peripheral, it would be desirable to manufacture the regulator using the same process technology as that used to manufacture devices in the peripheral. Generally, however, this is not possible, since the input load to the regulator can damage lower voltage devices.
The present invention solves the problem of using lower voltage devices with input voltages exceeding the maximum rated voltages of devices used in the voltage regulator.